Robust tests for log-logistic models based on minimum density power divergence estimators

Abstract: The log-logistic distribution is a versatile parametric family widely used across various applied fields, including survival analysis, reliability engineering, and econometrics. When estimating parameters of the log-logistic distribution, hypothesis testing is necessary to verify assumptions about these parameters. The Wald test and Rao test provide formal methods for testing hypotheses about these parameters. However, these test statistics are not robust, and their rejection decisions may be affected by data contamination. In this paper we develop new families of Wald-type test statistics and Rao-type test statistics based on minimum density power divergence estimators (MDPDEs) for the parameters of the log-logistic distribution. These new families generalize the Wald and Rao test statistics, inheriting the robustness properties from the MDPDEs and thus addressing the lack of robustness of the classical tests. Explicit expressions for the test statistics under the log-logistic model for both simple and composite null hypotheses are derived, and their properties are analyzed in detail. An extensive simulation study empirically demonstrates the robustness of these families and compares their performance with the classical methods.